Formation of ketols from linolenic acid 13-hydroperoxide via allene oxide. Evidence for two distinct mechanisms of allene oxide hydrolysis.

Incubations of [1-14C]13-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid (13-HPOT) with hydroperoxide dehydrase preparations from flax seeds lead to the formation of a novel ketol 2 along with the previously known 12-oxo-13-hydroxy-9(Z),15(Z)-octadecadienoic (12,13-alpha-ketol) and 9-hydroxy-12-oxo-10(E),15(Z)-octadecadienoic (gamma-ketol) acids. Compound 2 was identified as 11-hydroxy-12-oxo-9(Z),15(Z)-octadecadienoic acid (11,12-alpha-ketol) in accordance with the data of ultraviolet, mass (chemical ionization and electron impact) and 1H-NMR spectra. During long-term (30 min) incubations the yields of gamma-ketol and 11,12-alpha-ketol increased markedly and the yield of 12,13-alpha-ketol decreased in response to the pH change from basic (pH 7.4) to acidic (pH 5.8) conditions. Short-term (15 s) incubations of 13-HPOT with hydroperoxide dehydrase, terminated by HCl fixation, led to the formation of gamma-ketol and ketol 2. A similar incubation, followed by NaOH fixation, afforded only 12,13-alpha-ketol. The trapping of allene oxide (a primary product of hydroperoxide dehydrase) with pure methanol gives only compound 4 (12,13-alpha-ketol methyl ether). Products 5 (gamma-ketol methyl ether) and 6 (11,12-alpha-ketol methyl ether) were formed along with 4 as a result of trapping with acidified methanol. The results obtained indicate that: (a) the formation of 12,13-alpha-ketol is base-dependent; (b) the formation of gamma-ketol and ketol 2 is acid-dependent. Two distinct mechanisms of allene oxide hydrolysis are proposed: (1) nucleophilic (SN2 or SN1, OH- is an attacking group) substitution, resulting in formation of 12,13-alpha-ketol; (2) electrophilic (SE-like) reaction initiated by protonation of oxirane, affording gamma-ketol and 11,12-alpha-ketol.